Look at the source of facebook.com/facebook as it's sent to an iPhone. In the head is a tag called al:ios:url, and its value is fb://profile/100064860875397. Facebook's own page tells the app to open a long number, not the word "facebook" from the address. That's why the generator won't build a scheme from your link: a name-based address such as facebook.com/yourpage doesn't contain the number the scheme needs, so a scheme built from the name would open the app to nothing.
Two forms, and one the generator refuses
- The https link (
https://www.facebook.com/yourpage). Facebook's file for Apple lists the main Facebook app, and its Android counterpart lists com.facebook.katana. So the link opens the page or profile in Facebook, or loads it on facebook.com. - The custom scheme. Refused, for the reason above: a scheme needs the number, and your link only has the name.
- The Android intent. Only Chrome on Android acts on it: Facebook opens if it's installed, and the facebook.com address loads if not.
Where the number comes from
If you need fb:// for a button inside your own app, where Facebook is sure to be installed, look for the al:ios:url tag in your page's source. On Facebook's own page, that tag holds the fb:// address with the number. The generator doesn't fetch pages, so it can't look the number up for you. It only reads the shape of the link you paste.
The same tags, a different answer
Other platforms put the same al: tags in their page heads. Instagram's profile pages put an instagram:// address there. LinkedIn's profile pages put the plain https profile address there for both iPhone and Android. Facebook's page is the one that shows a number in the address, which is the case the generator is built to catch. See the LinkedIn deep link page and the Instagram page for their versions.
How to test it
- On a phone with Facebook installed, open the https link from an email and from a web page. Both should open the app.
- Tap it on a phone without Facebook. It should load the page on facebook.com in the browser.
The main deep link generator shows every form side by side for any supported link.